188 CHORD AT A. 



Class VI. Mammalia. 



Hair-clothed, warm-blooded vertebrates, viviparous with the excep- 

 tion of Monotremes ; milk-glands always present in the female. A 

 double circulation, membranous right auriculo-ventricular valve, single 

 aortic arch curving to the left, and non-nucleated red blood-corpuscles. 

 A complete muscular diaphragm (respiratory muscle). Two occipital 

 condyles, and seven (6-9) cervical vertebrae. 



Sub-Class I. PKOTOTHEKIA (Ornithodelphia). 



Order 1. Monotremata. A persistent cloaca, epipubic (marsupial) 

 bones and beak-like horny jaws. Oviparous. Ova large and cleavage 

 meroblastic. No placenta. Only three genera are known. Ornitho- 

 rhynchus, duck-billed platypus, Australia and Tasmania ; Echidna 

 (Tachyglossus), and Proechidna (Acanthoglossus), spiny ant-eaters, the 

 former in Australia, Tasmania, and North New Guinea, the latter in 

 South New Guinea. 



